Manganese-containing material treated by the invention can include manganese dioxide minerals in any form, including ores, or nodules, such as deep sea nodules.
Polymetallic or manganese nodules, are concretions formed of concentric layers of iron and manganese oxides around a core.
Deep sea nodules on the ocean floor include in their composition at least manganese (Mn) and usually nickel, cobalt, copper, zinc, and iron, with small amounts of titanium, vanadium, molybdenum, and cerium. Often present in addition are one or more of the following metals: magnesium, aluminum, calcium, cadmium, potassium, sodium, zirconium, titanium, lead, phosphorus, and barium.
All of the desired valuable metals in manganese nodules are tied up with insoluble oxidized manganese, such as MnO2. Only about 5% of the manganese contained in the nodules is acid soluble. Thus it is necessary to reduce the MnO2 by a suitable reducing agent as a first step in order to recover the metal constituents. Historically, SO2 has been used for this purpose. For deep sea nodules, carbon monoxide has also been used. However such prior art processes often do not recover a suitable manganese product, are capable of recovering only from about 80 to about 92% of the primary metal values, and often produce large quantities of waste. Moreover, a sulfate system requires large sized equipment with attendant high capital cost.
Unexpectedly, and contrary to earlier teachings, it has been found that reacting manganese-dioxide-containing materials with nitric oxide in the presence of nitric acid in aqueous solution results in recovery of at least 99% of the manganese values and associated metals (known as “pay metals” because they have higher commercial value than manganese).